Powder metallurgy is a technique for manufacturing a metal product or ingot by charging a metal or alloy powder into a mold, pressure-molding the powder, and sintering the molded product. Powder metallurgy is advantageous in that segregation of components does not occur, a product can be obtained from a material difficult to work, a member having a very fine crystal structure can be obtained, secondary machining can be omitted, and the like. These advantages cannot be obtained using a technique for manufacturing a metal product or ingot by melting a metal.
Three typical methods of manufacturing a powder of a high alloy, a Ti alloy, and the like will be described.
A. R. Cox, J. B. Moore, E. C. Van Reuth; Int. Symp. Superalloys, 3rd discloses a technique in which a metal is melted in a container by an RF current, the molten metal is dropped onto a disk rotated at a high speed, the dropped molten metal is scattered by a centrifugal force, and the scattered metal particles are rapidly solidified with a cooling medium having a high thermal conductivity such as hydrogen gas and helium gas.
G. Friedman; AGARD Conf. Proc., (1976) SCI discloses a technique in which an arc is generated between a nonconsumable electrode and a consumable electrode rotated at a high speed, the metal droplets generated by the molten consumable electrode are scattered by a centrifugal force, and the scattered metal droplets are cooled, thereby obtaining a metal powder.
H. Schmit; Powder Metall. Int. 11(1976) 1, p17 discloses a technique in which an arc is generated between a water-cooled crucible and an electrode to thermally melt the distal end portion of the electrode by the heat of the arc, the molten droplets dripped into the crucible being rotated at a high speed to scatter and cool them, thereby manufacturing a powder.
In the method of A. R. Cox, since the molten metal is reserved in the container, impurities can be mixed in the molten metal from the container. Therefore, a high-purity powder cannot be manufactured.
In the method of G. Friedmean, since the droplets are scattered by the centrifugal force obtained by the rotating consumable electrode, when a powder having a small particle size is to be obtained, the consumable electrode must be rotated at a high speed. However, it is quite difficult to rotate the electrode at a high speed because of the electrode machining precision and an electrode rotating mechanism. When the diameter of the electrode is decreased, the electrode can be rotated at a high speed. In this case, however, the lot scale is decreased, and electrode manufacturing costs per unit volume become expensive
In the method of G. Schmit, since the rotating crucible also serves as the nonconsumable electrode, it must be rotated at a high speed while it is energized. This is quite difficult because of the crucible machining precision and the crucible rotating mechanism. This problem becomes conspicuous when a metal powder having a small particle size is to be obtained because in this case a higher speed of rotation is needed.